This invention relates generally to mixer circuitry and more particularly to mixer circuitry having a local oscillator and mixer formed as a monolithic microwave integrated circuit (MMIC).
As is known in the art, mixer circuitry is used in a wide variety of applications. In one such application, the mixer includes a local oscillator for providing a radio frequency signal which, after amplification, is transmitted, via a suitable transmitter antenna, as a continuous wave (CW) radar signal. Energy reflected by an object in the path of the transmitted CW radar signal is fed, via a suitable antenna, to a mixer circuit. Also fed to the mixer is a portion of the continuous wave signal produced by the local oscillator. The mixer thereby produces an intermediate frequency signal having as a frequency component thereof a frequency related to the Doppler velocity of the object producing the reflected energy.
As is also known in the art, in order to reduce the size of such mixer circuitry it is desirable to increase the operating frequency of the local oscillator. One application requires that the oscillator produce a frequency in the millimeter wavelength region. One type of transistor adapted in the millimeter wavelength region is a heterojunction bipolar transistor (HBT). Such transistor may include a semi-insulating gallium arsenide (i.e., III-V material) substrate, an N+ type conductivity GaAs subcollector layer, an N type conductivity GaAs collector layer, a P+ type conductivity type base layer, an N type conductivity InGaP, or AlGaAs emitter layer and an N+ type conductivity type emitter contact layer sequentially formed as a single crystal body. As is also known in the art, it is desirable to form microwave circuitry as a monolithic microwave integrated circuit (MMIC) where both active devices, such as transistors, and passive devices, such as microstrip transmission lines, are also formed on the single crystal body. While such MMIC is desirable, such may not be practical to manufacture because one type of active device may not be readily compatible, from a manufacturing aspect, with other types of devices. That is, the two types of devices may not be readily formed with sufficient common processing steps to produce an MMIC which is economically practical. For example, while it is desirable to use Schottky diodes in the mixer circuitry discussed above, fabrication of the HBT oscillator of the mixer with sufficient processing steps common with the production of the Schottky diodes to result in an economical process has not been described.
As is also known in the art, in order to minimize the conversion efficiency of the mixer circuit, the diode used therein should be DC biased to some optimum operating point on its voltage-current (i.e., V-I) characteristic curve. However, the DC bias point varies with the power of the local oscillator signal. Therefore, because the power of the local oscillator varies with the operating temperature of the oscillator, the mixer will not operate with minimum conversion loss over the operating temperature of the mixer circuitry.